This invention concerns a device to detect surface faults in a metallic bar or wire rod in movement.
To be more exact, the invention concerns a device which detects disturbances in the electromagnetic field which are induced by the passage of a hot metallic bar or wire rod, for instance of a hot rolled type passing through a hot rolling mill, within a suitable measurement assembly.
The invention is applied advantageously to the iron and steel industry so as to achieve an automatic detection, in the rolling line, of faults such as cracks, flaking and microflaws on the surface of the hot rolled, drawn or extruded bar or wire rod. The invention can be used for speeds of the rolled stock up to 120-130 meters per second.
The state of the art includes a system for the automatic checking in line of the faults on the surface of substantially round rolled products such as metallic bars of a small diameter or wire rods.
This system makes use of the phenomenon of parasitic currents which are formed on the surface of a conductive body lapped by a variable magnetic flow.
The intensity of these parasitic currents is in proportion to the intensity of the excitation flow and to the surface resistivity of the material.
It is therefore possible to induce, as shown in EP-A-0449753 for instance, by means of an excitation coil an alternating magnetic flow on a metallic bar or wire rod to be inspected; this magnetic flow generates on the surface of the metallic bar a circulation of parasitic currents.
These parasitic currents in their turn produce on receiver coils arranged symmetrically in relation to, and in the vicinity of, the excitation coils a magnetic feedback flow, which, when the metallic bar and the axis of the coils lie substantially on the same axis and there are no faults on the surface of the metallic bar, is perfectly symmetrical in relation to the axis of the bar, to the first excitation coil and to the receiver coils.
If instead the metallic bar contains surface faults in the form of cracks, flaking or microflaws, the unbalancing of the parasitic surface currents entails an unbalancing of the relative feedback flow, which leads to unequal induced voltages.
If one of the two receiver coils is connected to the inverting input of a differential amplifier and the other receiver coil is connected to the non-inverting input of a differential amplifier, a signal of detection of the unbalancing of the feedback flows is obtained at the output of the amplifier.
So as to ensure conditions of maximum sensitivity and maximum uniformity of detection of the device, the element to be inspected has to be fed to the immediate vicinity of the receiver coils so as to minimise the effect of external disturbances which may be produced in the working environment.
This means that the detection head containing the receiver coils should be interchangeable so as to be adapted to the various ranges of diameter of the rolled bar.
Moreover, there is the danger that, if the rolled bar passes very near and contains any deformation, or is subject to an uncontrolled transverse vibration, the bar may be brought into contact with the coils and may cause damage or breakage of the coils or of the bar.
U.S. application Ser. No. 3,518,533 discloses the arrangement of a plurality of coils in series and their displacement in coordination during the passage on the bar to be inspected, so that the coils are brought to the vicinity of the bar; each coil acts on a determined sector of the bar so that all the coils together cover the whole surface to be inspected.
The mechanical displacement system disclosed is substantially unsuitable for bars travelling at the very high speeds usually employed in modern plants, and this leads to faults and inaccurate measurements.
Moreover, this system is substantially arranged for surface inspections of cold metallic wire rods or bars.
In the state of the art the bars are generally passed through the fault detection device substantially without being conditioned laterally apart from a rough alignment.
This solution entails the inability to bring the coils very near to the bar and also the occurrence of disturbances due to the bar being continuously off the axis of the device owing to the parallel traversing of the axis of the bar and to the inclination of the bar.
Instead, it is necessary that the amplitude of the disturbances should be reduced to a minimum so as to ensure a correct and efficient detection of the surface faults by the device.
There is therefore the problem of cancelling or at least restricting these transverse movements and vibrations of the bar in movement, which can lead to disturbances of the field on the receiver coils and may make the bar come into contact with the coils.
EP-A-0231865, EP-A-0096774 and U.S. application Ser. No. 3,582,771 disclose devices for the lateral clamping of bars moving through assemblies performing a non-destructive inspection of the bars. These disclosed devices, owing to their structure, are unable to cooperate efficiently with hot wire rods or bars travelling at a very high speed and are therefore substantially unsuitable for the plants commonly employed nowadays.